1. Field of the Invention
This invention relates generally to the management and treatment of wastewater and wastewater sludges, and is more particularly concerned with anaerobic treatment wherein the sludge is conditioned and recycled to the anaerobic reactor, or directed to dewatering and drying.
2. Description of the Prior Art
Many biological technologies have been first applied to wastewater treatment, and later used in other applications, sometimes related to the environmental technologies. Wastewater management and treatment methods and apparatuses are described in literature, for example, in the following sources:
Water and Wastewater Engineering, Vols. 1 and 2 by Gordon Maskew Fair, John Charles Geyer and Daniel Alexander Okun, John Wiley & Sons, 1958; PA1 Biological Waste Treatment by Wesley W. Eckenfelder and Donald J. O'Connor, Pergamon Press, 1961; PA1 Water Preparation for Industrial and Public Water Supplies by A. A. Kastalsky and D. M. Mints, Publishing House Higher Education, Moscow, 1962 (Russian); PA1 Treatment of Natural Waters by V. A. Klyachko and I. E. Apeltsin, Publishing House Stroyizdat, Moscow, 1971 (Russian); PA1 Physicochemical Processes by Walter J. Weber, Wiley-Interscience, New York, 1971; PA1 "Anaerobic Waste Treatment Fundamentals" by Perry L. McCarty, Public Works, pp. 107-112, September 1974, pp. 123-126, October 1974, pp. 91-94, November 1974, pp. 95-99, December 1974; PA1 Biochemical Treatment of Wastewater from the Organic Chemicals Manufacturing by F. V. Porutsky, Moscow, Publishing House Khimiya, 1975 (Russian); PA1 Chemistry for Environmental Engineering by Clair N. Sawyer and Perry L. McCarty, McGraw-Hill, 1978; PA1 Metcalf & Eddy's Wastewater Engineering Vols. 1 and 2, Edited by George Tchobanoglous, McGraw-Hill, 1979; PA1 Biological Process Design by Larry D. Benefield and Clifford W. Randall, Prentice Hall, 1980; PA1 Water Chemistry by Vernon L. Snoeyink and David Jenkins, John Wiley & Sons, 1980; PA1 Biological Wastewater Treatment by C. P. Leslie Grady and Henry C. Lim, Marcel Dekker, Inc., 1980; PA1 Low-Maintenance, Mechanically Simple Wastewater Treatment Systems by Linvil G. Rich, McGraw-Hill Book Company, 1980; PA1 Biochemical Processes in Wastewater Treatment by S. V. Yakovlev and T. A. Karyukhina, Stroyizdat, Moscow, 1980 (Russian); PA1 Handbook on Design of Wastewater Treatment Systems, Edited by V. N. Samokhin and Boris M. Khudenko, Allerton Press, New York, 1986; PA1 Treatment of Wastewater Sludges by I. S. Turovskyi, Stroyizdat, Moscow, 1988 (Russian); PA1 Utilization of Wastewater Sludges by A. Z. Evilevich and M. A. Evilevich, Stroyizdat, Sankt Peterburg, 1988; PA1 Industrial Water and Wastewater Systems by S. V. Yakovlev, Ya. A. Karelin, Yu. M. Laskov, Yu. V. Vorononv, Publishing House Stroyizdat, Moscow, 1990 (Russian); PA1 Design of Anaerobic Processes for the Treatment of Industrial and Municipal Wastes Edited by Joseph F. Malina and Frederick G. Pohland, Technomic Publishing Co., 1992. PA1 single predominantly aerobic sludge is formed in the system, such sludge having a poor diversity of species and a narrow range of oxidation-reduction and biodegradation ability; PA1 process can be used only for dilute to moderately strong wastewater; PA1 sludge concentration along the process train and along major process zones is almost uniform; PA1 F/M ratio in various process zones varies drastically; PA1 in the downstream sections, the wastewater concentrations are low, while the sludge concentration is about the same as upstream; accordingly, sludge dies off from lack of food, releasing nitrogen, phosphorus, and organics back into the water; PA1 sludge generation by mass and volume is high, so the sludge disposal costs are high; PA1 sludge age is high and so is the corresponding degree of sludge stabilization; PA1 at high sludge stabilization, the content of organics anaerobically convertible to methane is low and so is the sludge mass and volume reduction in this conversion; PA1 degradation of soluble organics is poor due to limited oxidation-reduction potential (OPR) range, especially xenobiotic, recalcitrant or poorly degradable organics (halogenated, and others); PA1 usually, the SS content in the influent to the ASP process is limited by about 100 mg/l, otherwise removal of suspended solids is poor; PA1 process stability in response to dynamic overloading and toxic shocks is low; PA1 volatile organics may be emitted to the air in facultative, anoxic and aeration sections. PA1 almost uniform sludge make-up and concentration along the major process zones (poor F/M ratios in various process zones), and poor diversity of species in the sludge in each functional section; PA1 operational difficulties in treating low concentration wastewater; PA1 high sludge age and high degree of sludge stabilization in the aerobic subsystem (low content of organics convertible to methane and low mass and volume reduction in such conversion); PA1 poor removal of suspended solids; PA1 low process stability in response to dynamic overloading and toxic shocks; PA1 low efficiency of degradation of poorly and slowly degradable and toxic organics; PA1 loss of volatile organics to the air in open facultative, anoxic, and aeration sections; PA1 difficulties in removing nutrients (nitrogen and phosphorous).
Various fundamental and practical aspects of the relevant water and wastewater management and treatment processes are described in the above listed sources. These data are also applicable to other processes, for example, conversion of solid and liquid waste and other materials into biogas and biological fertilizers and soil augmentation substances.
The generally accepted wastewater management method comprises steps of collecting wastewater in a system of pipes and channels, transporting it by these pipes and channels to a treatment works, treating it at the said treatment works, discharging the treated effluent into natural bodies of water or on land, or reusing it for water supplies.
The existing wastewater management systems have the following disadvantages:
1. Under anaerobic conditions in the collection pipelines, wastewater in the bulk flow becomes acidified. Volatile and nonvolatile fatty acids are formed, and sulfates are substantially reduced to sulfides, while fatty acids are partially converted to methane. At the gas-water interface in the pipes, sulfuric acid is formed. Therefore, the processes in pipelines can only cause the formation of odorous, poisonous, ignitable and explosive gases, and corrosion of pipes. Similar problems occur at the front end of wastewater treatment plant. Sometimes odor problems may become severe. PA0 2. The wastewater treatment systems are complex, energy demanding, and expensive to build and operate. Improvements to the wastewater treatment facilities is an ongoing process; however, these improvements are separate from the improvements in the collection and separation networks. PA0 1. Only wastewater with simple soluble substrate (easily degradable nontoxic constituents) can be adequately treated anaerobically. PA0 2. Suspended solids in the wastewater influent are not satisfactorily degraded unless retention time in the reactor is very long (usually 3 to 15 days or longer). Long retention time requires excessive reactor volumes as in low rate processes, which are difficult to mix well, and therefore, breakthroughs of pockets of poorly mixed and poorly treated waste occur. This reduces average efficiency of treatment. PA0 3. Slowly and poorly degradable, or toxic, soluble constituents of the wastewater influent are not degraded unless retention time in the reactor is very long, or a bed of granular activated carbon (GAC) is provided. In the latter case, a portion of the GAC bed must be periodically replaced due to the accumulation of nondegraded adsorbed material. PA0 4. Liquid in anaerobic reactors often turns acidic due to the accumulation of fatty acids. This can be caused by an overloading with organics, or by a toxic effect of specific constituents in the feed or by poor mixing in low rate processes. Accumulation of fatty acids and the respective drop in pH cause depletion in the methanogenic population. Further accumulation of fatty acids may cause suppression in the growth of acidogens. Inadequate growth of either group of organisms results in a process upset. There are no means for controllable cultivation, maintenance, accumulation and use of major groups of organisms in the prior art anaerobic systems. Since methanogenic organisms have very slow growth rate, the anaerobic process recovery takes a long time. This problem becomes especially difficult during start-up operations because acidity control requires large quantities of alkalies, and the start-up process may last many months, and sometimes a year or longer. Process controls, except pH correction with reagents, are not provided PA0 5. Toxic discharges (for example, slugs of acidic or alkalinic wastewater, or wastewater having elevated concentrations of toxic constituents) can poison the entire sludge population in the reactor, thus requiring a long restarting time. PA0 6. Either thermophilic (about 55.degree. C.) or mesophilic (about 33.degree. C.) are used. At temperatures lower than mesophilic, the process rate becomes very slow. PA0 7. Sludge concentration in the suspended growth processes is low, usually from 10 to 60 g/l . Accordingly, substantial effort is required to dewater sludges by using centrifuges, vacuum filters, filter presses or other expensive methods. PA0 8. Anaerobic processes are not intended for controlling nutrients and heavy metals. PA0 9. Anaerobic processes generate odorous gases such as hydrogen sulfide, and volatile organics. Accordingly, gases need to be collected, even at small treatment plants, and are usually treated and/or combusted. PA0 10. Anaerobic reactors for wastewater treatment have deficient systems for water distribution, gas collection, and sludge separation. Foam and scum often are accumulated in the upper sections of anaerobic reactors. Poor mixing in low rate systems reduces the treatment efficiency. PA0 11. Anaerobic systems for wastewater and sludge treatment have no process controls beyond temperature correction with heating and pH correction by reagents. Poor mixing in low rate processes makes automation difficult because of a resulting random nature of concentrations distributions in the reactors. PA0 12. Anaerobic reactors require a large area, because structural and cost considerations limit the total reactor height to 6 to 9 meters. Special egg-like shapes for avoiding grit accumulation are complex and costly to erect. PA0 1) Sludge is bioheated before bioflotation, which increases the process rate and insures bioflotation in colder climates. Bioflotation is achieved by subjecting the sludge to anaerobic conditions wherein methane, carbon dioxide, and/or nitrogen are preferably formed. In some instances, hydrogen sulfide may also be formed. These gases float up the sludge. PA0 2) Nitrates and nitrites are generated from nitrogen sources (urea or ammonia) or added to and mixed with the aerobically bioheated sludge, or immediately after the bioheating step in order to promote, respectively, denitrification and sludge flotation. Nitrates accelerate the process described in previous paragraphs. PA0 3) The sludge flow is split into parallel aerobic bioheating and anaerobic digestion steps. In the anaerobic step, long sludge ages are maintained to cultivate denitrifying acidogenic and methanogenic bacteria. Methanogenic and denitrifying bacteria consume fatty acids generated by the acidogenic organisms. At a longer sludge age, methanogenic and denitrifying bacteria deplete the fatty acids and other organic sources required for the growth of sulfate reducing bacteria. Accordingly, the growth of sulfur reducers is suppressed, the hydrogen sulfide is generated in very small quantities, and the process can be kept substantially odor free.
Several methods for controlling anaerobic processes in the sewer networks have been used: providing oxidative environment, for example by ventilation of the pipes with air, or by addition of other oxidants; by depressing the growth of sulfate reducing organisms with chemicals effecting specific biochemical steps; or, by raising wastewater pH. All such measures add to the cost of wastewater management and are not focused on wastewater treatment.
Several modifications of wastewater treatment processes have been developed: 1. aerobic (activated sludge process, lagoon systems, and biofiltration), 2. anaerobic (various attached and suspended growth processes), and 3. coupled anaerobic-aerobic systems. Modern biological treatment systems are used for removal of organics and suspended solids, and for control of nutrients. However, these processes do not achieve thorough removal of organics, especially when measured in COD or TOC units, and removals of nitrogen and phosphorous are marginal. The prior art technologies do not provide controls over the balances of organics, nutrients, biomass, and other constituents of wastewater. In suspended growth aerobic systems, sludge recycle from the final sludge separator to the head of the treatment process is provided. These systems often incorporate several functional zones, usually called anaerobic (nonaerated, preferably, with low nitrate and nitrite in the feed), anoxic (nonaerated, nitrite and nitrate present in the feed water) and aerobic (aerated, dissolved oxygen present in the water, nitrification occurs). Mixed liquor is recycled from downstream zones to upstream zones and the separated activated sludge is recycled from the final clarifier to the head of the process. A so-called single sludge is cultivated in all these zones. This is predominantly aerobic sludge. It includes very few strictly anaerobic organisms. Facultative anaerobic organisms develop in the nonaerated zone; therefore, the nonaerated zone in these systems should be more properly called the facultative zone. This term will be used in this application. The sludge recycle from the final clarifier is intended mainly for controlling the average sludge age, or average for the system food to microorganism (F/M) ratio. The upstream facultative zone serves to control the filamentous growth (selector zone) and to release phosphorous for, as believed, its improved uptake in the aerobic zone. The facultatively anaerobic organisms are circulated with the sludge throughout the system. Anoxic zones are used for denitrification: the biological reduction of nitrites and nitrates formed in the aerobic zone and directed to the anoxic zone with the mixed liquor. These systems are used for treatment of municipal and low to moderately strong industrial wastewater. Examples of these systems are described in U.S. Pat. No. 3,964,998 and U.S. Pat. No. 4,867,883. The disadvantages of such systems include the following:
Anaerobic treatment of wastewater and wastewater sludges is well known in the art. In the past this technology was used mainly for sludge digestion and for simplified treatment of small wastewater streams in septic tanks. Recently, the anaerobic method has been applied to treat larger flows of a more concentrated industrial wastewater, primarily in the food and beverage industries. These more recent applications have revealed general advantages and disadvantages of anaerobic treatment methods. Additionally, fundamental research has been conducted on treatment of more complex wastewater, including industrial wastewater samples and imitations thereof with poorly degradable and toxic organics. This research demonstrated additional capabilities, advantages and problems associated with anaerobic processes. The present status of anaerobic treatment technologies is very thoroughly described in a recent book, Design of Anaerobic Processes for the Treatment of Industrial and Municipal Wastes, edited by J. F. Malina and F. G. Pohland, Technomic Publishing Inc., 1992. Additionally, in 1992-1993 the applicant conducted a study of anaerobic treatment of a complex wastewater, which is used in this application to demonstrate advantages of the new and improved method.
Two major anaerobic treatment methods were developed in the past: (1) attached growth processes; and, (2) suspended growth processes. Some modifications are classified as hybrids of these methods. Advantages and disadvantages of prior methods are given in the above mentioned book. The major advantages of anaerobic systems are the low energy requirements, with potentially a net generation of energy, and a relative simplicity of treatment units and operations. Disadvantages of prior anaerobic treatment systems are summarized as follows:
In summary, the above mentioned problems numbered 1 to 11 are related to a deficient sludge management strategy in prior art anaerobic wastewater treatment systems, and problems numbered 9 to 12 are related to deficient designs of anaerobic reactors. These two fundamental deficiencies limit the use of anaerobic treatment systems and cause operational problems in many of the systems already built.
The coupled anaerobic-aerobic systems have been developed and used during the past fifty years for treatment of concentrated industrial wastewater. These systems incorporate a separate anaerobic subsystem (functional section) with the final anaerobic clarifier and sludge recycle, and aerobic subsystem separation and sludge recycle step. Only excess aerobic sludge may sometimes be transferred to the anaerobic subsystem. This system has important advantages as compared to aerobic systems: high concentration waste can be treated, lesser quantities of sludge are produced, and better removal of soluble and suspended solid organics can be achieved.
However, anaerobic and aerobic functional sections in the anaerobic-aerobic systems are only mechanistically coupled. Sludges in these sections do not interact: their make-up and properties abruptly change from anaerobic to aerobic stage. The major disadvantages of anaerobic-aerobic systems are as follows:
Several modifications of biofiltration systems have been developed, including aerobic and anaerobic, with and without water recirculation, a single, or multiple-stage system. Various lagoon systems have also been developed. Most often the lagoon systems comprise a series of aerated or nonaerated sections. Hydraulic retention time in lagoons is very long and sludge recycle is not practiced. Processes in lagoons are usually similar to those in ASP, but are not intensive and are less controlled. Some lagoons may have an anaerobic section, often followed with an aerobic sections. Such lagoons are similar to the anaerobic-aerobic systems. Open anaerobic lagoons produce odors. Large water volume in the systems insures equalization of wastewater and sludge concentrations and provides a substantial process stability. However, poor mixing causes breakthroughs of poorly treated waste, and an overall low process efficiency. These systems are mechanically simple and require low maintenance. Many disadvantages of ASP and anaerobic-aerobic processes listed above are also typical for biofilters, rotating biological contractors, lagoon systems and various other modifications of biological wastewater treatment.
Sludges generated in wastewater treatment processes, for example in biofiltration or activated sludge process, are usually directed for either aerobic or anaerobic biological stabilization. Sludge thickening may precede biological stabilization. Methods of sludge thickening include: gravity thickening in tanks designed as settling tanks, sometimes with gentle mixing; pressure air flotation; thermal gravity thickening/flotation thickening; vibratory filters; drum screens; and centrifuges.
During biological stabilization, sludge is substantially mineralized and becomes nonrotting; however, it retains a large proportion of water, which makes sludge disposal difficult. Accordingly, sludges are usually dewatered and dried, which may be accomplished on drying beds--the method preferred at smaller plants. Separate dewatering and drying are used at larger plants, the methods including vacuum filtration, filter pressing, centrifugation, etc. Separate methods of drying include drying beds, rotary drums, fluidized bed dryers, dryers with opposite jets, etc. Sometimes sludges are thickened, dewatered and dried without biological stabilization, or a chemical stabilization is used instead.
Thermal gravity thickening, and thermal gravity/flotation thickening show significant advantages over other thickening methods. These methods are described in the book Utilization of Wastewater Sludges, by A. Z. Evilevich and M. A. Evilevich, Publishing House Stroyizdat, Leningrad (S. Peterburgh), 1988 (in Russian) and in Soviet Certificates of Invention Nos.: 300420, 1971; 381612, 1973; 1118623, 1984. Advantages include more rapid and more efficient separation (thickening) of sludge particles from water. A major disadvantage of these methods is in that heating of the sludge prior to the separation is done by a heat carrier, for example steam, which requires additional complex equipment, heat exchangers or the like, and energy from an external sources (such as fuel). Sometimes flotation is not stable and portions of the sludge hang up in the mid depth or settle to the bottom of the flotation tank. Additionally, odor due to generation of hydrogen sulfide often occurs.